Mineral oil based lubricants are conventionally produced by a separative sequence carried out in the petroleum refinery which comprises fractionation of a paraffinic crude under atmospheric pressure followed by fractionation under vacuum to produce distillate fractions (neutral oils) and a residual fraction which, after deasphalting and severe solvent treatment may also be used as a lubricant base stock usually referred as a bright stock. Neutral oils, after solvent extraction to remove low viscosity index (V.I.) components are conventionally subjected to dewaxing, either by solvent or catalytic dewaxing processes, to the desired pour point, after which the dewaxed lube stock may be hydrofinished to improve stability and remove color bodies. This conventional technique relies upon the selection and use of crude stocks, usually of a paraffinic character, which produce the desired lube fractions of the desired qualities in adequate amounts. The range of permissible crude sources may, however, be extended by the lube hydrocracking process which is capable of utilizing crude stocks of marginal or poor quality, usually with a higher aromatic content than the best paraffinic crudes. The lube hydrocracking process, which is well established in the petroleum refining industry, generally comprises an initial hydrocracking step carried out under high pressure in the presence of a bifunctional catalyst which effects partial saturation and ring opening of the aromatic components which are present in the feed. The hydrocracked product is then subjected to dewaxing in order to reach the target pour point since the products from the initial hydrocracking step which are paraffinic in character include components with a relatively high pour point which need to be removed in the dewaxing step.
Current trends in the design of automotive engines are associated with higher operating temperatures as the efficiency of the engines increases and these higher operating temperatures require successively higher quality lubricants. One of the requirements is for higher viscosity indices (V.I.) in order to reduce the effects of the higher operating temperatures on the viscosity of the engine lubricants. High V.I. values have conventionally been attained by the use of V.I. improvers e.g. polyacrylates, but there is a limit to the degree of improvement which may be effected in this way; in addition, V.I. improvers tend to undergo degradation under the effects of high temperatures and high shear rates encountered in the engine, the more stressing conditions encountered in high efficiency engines result in even faster degradation of oils which employ significant amounts of V.I. improvers. Thus, there is a continuing need for automotive lubricants which are based on fluids of high viscosity index and which are stable to the high temperature, high shear rate conditions encountered in modern engines.
Synthetic lubricants produced by the polymerization of alpha olefins in the presence of certain catalysts have been shown to possess excellent V.I. values, but they are expensive to produce by conventional synthetic procedures and usually require expensive starting materials. There is therefore a need for the production of high V.I. lubricants from mineral oil stocks which may be produced by techniques comparable to those presently employed in petroleum refineries.
It is well known that alpha olefins useful in the preparation of synthetic lubricants can be produced by the ethylene growth reactions or by cracking petroleum waxes, including slack wax. Typically, the products of ethylene growth reaction or wax cracking are separated by distillation to recover the C.sub.10 fraction known to be especially useful in the production of the sought for high VI synthetic lubes. Oligomers of 1-alkenes from C.sub.6 to C.sub.20 have been prepared with commercially useful synthetic lubricants from 1-decene oligomerization yielding a distinctly superior lubricant product via reduced chromium, cationic or Ziegler catalyzed polymerization.
Discovering exactly those alpha olefins, and the associated oligomerization process, that produce a preferred and superior synthetic lubricant meeting the specification requirements of wide-temperature fluidity while maintaining low pour point represents a prodigious challenge to the workers in the field. Brennan, Ind. Eng. Chem. Prod Res. Dev. 1980, 19, 2-6, cites 1-decene trimer as an example of a structure compatible with structures associated with superior low temperature fluidity wherein the concentration of atoms is very close to the center of a chain of carbon atoms. Also described therein is the apparent dependency of properties of the oligomer on the oligomerization process, i.e., cationic polymerization or Ziegler-type catalyst, known and practiced in the art. While theoretical considerations abound as to the relationship between alpha olefin structure and the lubricant properties that will ensue from oligomerization thereof, the art is, as yet, unpredictable and a relatively expensive 1-alkene, i.e., 1-decene, is commercially relied upon to provide high VI synthetic lubricant.
In U.S. Pat. No. 4,395,578 to Larkin, a process is described for the oligomerization of alpha-olefins using boron trifluoride catalyst wherein the alpha-olefins are produced by ethylene polymerization or wax pyrolysis. Viscosity indices up to 112 are achieved.
In U.S. Pat. No. 4,420,646 to Darden et al, a process is described for the production of synthetic lubricants by the oligomerization of alpha-olefins produced from wax pyrolysis. Boron trifluoride catalyst providing viscosity indices of about 130 are reported. These processes do not disclose the oligomerization of the reaction mixture from slack wax cracking.
It is an object of the present invention to prepare high viscosity index lubricants from inexpensive refinery hydrocarbon products.
It is another object of the invention to prepare such lubricants that also exhibit low pour point and superior thermal stability.
A further object of the invention is to prepare such lubricants using slack wax as the feedstock.
Another object of the invention is to prepare such lubricants in high yield by the catalytic oligomerization of the product mixture of olefins recovered from thermally cracking slack wax.